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How Crash Recovery Works with Online DDL

Although no data is lost if the server crashes while an alter table statement is executing, the crash

recovery process is different for clustered indexes and secondary indexes.

If the server crashes while creating an InnoDB secondary index, upon recovery, MySQL drops any

partially created indexes. You must re-run the alter table or create INDEX statement.

When a crash occurs during the creation of an InnoDB clustered index_t recovery is morę complicated,

because the data in the table must be copied to an entirely new clustered index. Remember that all

InnoDB tables are stored as clustered indexes. In the following discussion, we use the word table and clustered index interchangeably.